1. Field of the Invention
The present invention relates to a method for producing an organic electroluminescence element and to an organic electroluminescence element produced by the method.
2. Description of the Related Art
In order for organic electroluminescence elements to be widely put into practical use, development has been demanded for a high-performance organic electroluminescence element whose drive voltage is low, whose service life is long, and whose durability is good.
In the production of an organic electroluminescence element, an electrode (e.g., an ITO film) constituting the organic electroluminescence element is generally subjected to surface treatment through oxygen plasma treatment or UV-ozone treatment. By virtue of this treatment, organic matter on the electrode is decomposed and washed away. As a result, no barrier with respect to HOMO level can be attained in an organic film on the electrode, leading to decrease in necessary drive voltage. Without surface treatment, stain on the electrode surface is not removed, potentially resulting in that sufficient durability cannot be obtained, for example, a short circuit occurs between the electrodes, and ununiform light emission is observed on a light-emitting surface.
However, when the electrode is subjected to surface treatment using oxygen, sufficient durability cannot be necessarily attained. In view of this, there is proposed surface treatment methods using oxygen-free gas such as argon gas (see Japanese Patent Application Laid-Open (JP-A) Nos. 2000-133064, 2000-512795 and 2000-150172).
For example, JP-A No. 2000-133064 discloses that, using an inert gas whose energy level falls within a range of 20 eV to 100 eV, an ITO film is irradiated with cations for surface modification.
Also, JP-A No. 2000-512795 discloses an electric device containing a semiconductive or conductive layer with a work function which is modified through a plasma treatment.
Also, JP-A No. 2000-150172 discloses that, in an organic EL display element having an organic illuminant-containing organic light-emitting layer between an anode and a cathode, at least one element selected from nitrogen, sulfur, selenium, tellurim, phosphorus and halogen is formed into plasma in the surface of the anode for surface treatment.
However, the elements formed through the surface treatments disclosed in these literatures disadvantageously require a higher drive voltage and involve a greater increase in voltage during driving than those formed through surface treatment using oxygen. In particular, when a reducing gas such as hydrogen gas and deuterium gas is used, the work function of the electrode becomes small. As a result, the hole-injection barrier with respect to an organic film becomes large, resulting in that the formed element requires a high drive voltage.
In one known method of reducing the drive voltage of an element, the organic film thereof is p-doped. When the electrode of the element having a p-doped organic film is oxygen plasma treated, the formed element is decreased in durability as described above. Without surface treatment, a short circuit occurs between the electrodes, and ununiform light emission occurs.
Thus, at present, there cannot still be obtained a satisfactorily high-performance organic electroluminescence element whose drive voltage is low, whose service life is long, and whose durability is good.